I am adding a 1/2 bath to the upstairs of our 170 year old cape and am considering the best route to run my PEX supply lines. Because there have been several additions and alterations to the house over the years, there are oddities in the structure that make it difficult to describe my situation here. But basically, the room where we are putting the 1/2 bath is in a small dormer at the end of the main portion of the house (what used to be the outside wall). Today, there is an addition on this end, with an unheated attic (the attic has a full dormer so you can stand up in it) which extends 25 feet to another barn portion of the house. At the far end of the attic, there is a 12"x12" channel running up from the basement which contains the 3" soil stack. The floor of the attic drops down about 18” from the floor of the small dormer where we want to put the new bath. So my plan is to come out of the small dormer into the attic, run the 25 feet, connect to the existing soil stack, and run down the channel to the basement with my PEX. I was planning to wrap all of these runs with plenty of fiberglass insulation, and “box in” the whole thing with plywood to keep it from freezing (I live in central New Hampshire so it gets cold). So if you could follow all of that, does this sound like a viable plan for both the PEX and the PVC runs? Can I run these through an unheated space? I really have no other choice for the 3” PVC but I could possibly come up with another route for the PEX.

Thanks in advance for your input!

You're asking a Florida plumber who hasn't seen snow in over fifty years how to keep pipes from freezing? OK, so I spent the first 28 of them in Wisconsin.
I'm sure you have the layout of your attic in your minds eye but it's not that simple for me. Is the 12" chase on a outside or inside wall? Heat raises from below and will mitagate the cold in the unheated portion of the attic if it's heated below it. This 25 foot run will go through the unheated section, correct? Is the unneated section of the attic over a heated section of the house? Back to you, Tom

Hi Tom!

Yes... The 25 foot run will go through the unheated attic, and the unheated attic is above our kitchen area (which of course is heated). The 12" chase is on an outside wall.

If you insulate the water line and run it on the inside wall of the chase next to a heated section you should be OK. If there's another way to get the water up stairs inside the house I would look into it but the way you've explained it it should work. Good luck, Tom,

Thanks... there are a couple of alternatives to run the water lines. One involves opening up a 1st floor wall which rests over a stone foundation in the basement, and the other involves coming up through the downstairs bathroom closet into the attic and drilling through a few ceiling joists (if I came straight up through the chase I would be inside a joist cavity which runs lengthwise to my destination). Maybe I should take a closer look at the bathroom closet route. I was more concerned about running them through the open attic than up through the chase. There is a floor in the attic so I would be over the kitchen ceiling which is insulated, and basically would be subject to outside temperatures. I'm also concerned about residual water in the PVC line freezing and building up over time. If I insulate the lines really well, than aren't I also insulating them from any heat from the kitchen which might keep them from freezing? I suppose I could pull up the attic floor to get my supply lines under the insulation and next to the warm kitchen ceiling, but think I will need to stay above the joist cavity to get my pitch on the pvc waste line.

I would be more concerned about the water lines then the drainage. The drainage lines stay comparably dry until a discharge runs through them, and running water will not freeze.

You ask," If I insulate the lines really well, than aren't I also insulating them from any heat from the kitchen which might keep them from freezing?"

Good question, but I wasn't concerned about insulating them from the heat as much as from the cold. I was counting on the ambient temperature radiating from the house to the chase to mitigate the cold and the insulation to do the rest. The upstairs attic has to be really well insulated to prevent some heat from raising up there. Cheers, Tom

Great! Sounds like a plan then. Thanks for your help!